LTF Technology Simplifies Light Sensing

A light-to-frequency (LTF) converter measures light intensity and produces a square wave signal with frequency directly proportional to the light intensity. The square wave's amplitude does not change with light intensity; only the frequency changes. The analog-to-digital conversion of the photodiode current to a signal that a microcontroller can use is performed inside the LTF converter.

Ray King, TAOS

06/01/2007

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A light-to-frequency (LTF) converter measures light intensity and produces a square wave signal with frequency directly proportional to the light intensity. The square wave's amplitude does not change with light intensity; only the frequency changes. The analog-to-digital conversion of the photodiode current to a signal that a microcontroller can use is performed inside the LTF converter. Using averaging techniques, sixteen bits of true resolution can be achieved with a LTF converter.

LTF converters are semiconductor devices that produce a squarewave output whose frequency increases with light level over as much as six orders of magnitude. Brighter light leads to higher frequency and longer wave period.

The converter consists of a photodiode that converts light intensity to a current, followed by a current-to-frequency converter that produces the frequency output. The square wave can be input directly to the timer/counter port of a microcontroller for digital processing.

The choice of measurement technique with a LTF converter depends on the desired resolution and data-acquisition rate. For maximum data-acquisition rate, period-measurement techniques are used. Period measurement requires the use of a fast reference clock with available resolution directly related to the reference-clock rate. The technique measures rapidly varying light levels or fast measurements of a constant light source.

High resolution and accuracy may be obtained using frequency-measurement, pulse-accumulation, or integration techniques. Frequency measurements provide the added benefit of averaging random or high-frequency variations (jitter) resulting from noise in the light signal. Resolution is limited mainly by the available counter registers and allowable measurement time. Frequency measurement is well suited for slowly varying or constant light levels and for reading average light levels over short periods of time. Integration, the accumulation of pulses over a very long period of time, can be used to measure exposure — the amount of light present in an area over a given time period.

A LTF converter measures light intensity and provides the data directly to a microcontroller in digital format. An alternative light sensing solution would include a photodiode, a transimpedance amplifier, passive components, and an analog-to-digital converter.

Averaging noise and flicker — Fluorescent lights flicker at twice the ac power frequency—120 Hz in the United States, and 100 Hz in many other countries. This flicker creates frequency “jitter.” Counting pulses with a microcontroller over a period of time and calculating the average frequency removes ac flicker and any other noise from the measurement.

The LTF converter chip replaces several discrete components.

Low susceptibility to noise—The LTF signal is digital throughout its path. In a traditional solution, the current from a photodiode is very small (microamps), making it susceptible to picking up noise, particularly if the transimpedance amplifier is separated from the photodiode by a considerable distance. In some applications, it is necessary to add shielding around the photodiode to keep electromagnetic interference (EMI) and radio frequency interference (RFI) from interfering with the signal. However, additional shielding is not needed with a LTF converter as long as adequate power supply decoupling is provided.

Direct interface to the microcontroller timer/counter port —Additional components, such as op-amps, or analog-to-digital converters are not needed to couple to the microcontroller. This translates into board space savings, and, since the entire circuit is on a single monolithic device, there is less opportunity for EMI and RFI to interfere with the measurement results.

LTF converters, however, do have some disadvantages:

Counting pulses over an extended period of time averages the LTF output frequency, suppressing any noise in the light signal.

Slow response to sudden light-intensity changes—The step response to a change in light intensity for the TAOS TSL230R is one period of the new output frequency plus 1 microsecond. This effect is exaggerated at low light levels since the output signal's period is proportional to the light intensity.

Slow measurements in very low light —Measurements in very low light require a relatively long time because the output frequency is low. A long output period means fewer samples can be taken in a given period of time. This leaves the tradeoff of slow measurement time versus resolution and noise averaging.

LTF converters are not a good choice, therefore, for applications in very low light levels unless measurement time is not a primary concern. Also, LTF converters are not a good choice for light-sensing applications requiring fast measurements, for measuring rapid light changes, or for applications in low light levels needing fast measurements, such as in money detectors.

They are, however, excellent for applications requiring a wide dynamic range, such as automobile lighting sensors and other applications requiring measurements of lighting ranging from as dim as moonlight to as bright as sunlight.

Another area where LTF converters excel is where accuracy and resolution are more important than speed, such as pulse oximetry and heart rate monitors. In pulse oximetry, LTF converters have the added benefit of high noise immunity.

LTF converters are also useful in commercial applications, such as monitoring office lighting, that exploit their ability to remove ac flicker and turbidity sensors for dishwashers to determine when dishes are clean.